DS2155 datasheet(178/262 Pages) DALLAS

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DS2156

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HEC error correction is disabled, all HEC-errored cells are termed as uncorrectable HEC-errored cells. A

16-bit count of the number of cells that can be written into receive FIFO is maintained, which saturates at

FFFFh. Note that, whether or not ATM layer dequeues cells from the receive FIFO, this counter

increments if valid cells are received. This counter is cleared by the microprocessor interface once it is

latched. A 2k x 8 receive FIFO that holds 4-cell buffer space per port is maintained for rate decoupling.

24.6.2

UTOPIA Side Receive: Muxed Mode with One Receive CLAV

The DS2156 drives the internal cell-available signals onto the external CLAV lines based on the

configured polling mode. In direct status mode, only four ports are supported. If a port is conducting a

cell transfer, its CLAV is kept asserted until the last byte is transferred to the ATM layer. This supports

interfacing with the octet-level ATM layer also. Cell-available status for any fresh cell corresponding to a

port has to be polled by the ATM layer only after the current cell transfer to the port is completed. The

multiplexed bus with 1CLAV polling mode cycle is depicted in Figure 24-9 in which N, N + 2, N - 3, N -

2, N - 1, N + 3, and N + 1 are considered part of DS2156 UTOPIA ports. During reception of a cell from

PHY N, the other PHYs are polled. It turns out that PHY N - 3 and PHY N + 3 have cells available, and

PHY N + 3 is ultimately selected. (Remember that the PHY number values are for example only). Just

like the transmit interface, the 2-clock polling cycles allow up to 26 PHYs to be polled in the 8-bit mode.

Figure 24-9. Polling Phase and Selection at Receive Interface

N+2

N-3

N-2

N-1

N+3

N+1

N-1

N+3

N+1

N-1

N+2

N-3

N-2

N-1

N+3

N+1

N+3

N+1

P35 P36 P37 P38 P39 P40 P41 P42 P43

P45 P46 P47 P48

P44

UR-CLK

UR-ADDRx

UR-CLAV

UR_ENB

UR-DATAx

UR-SOC

CELL RCV FROM:

PHY N

PHY N+3

POLLING

SELECTION

N-1

DS2155 Datasheet(PDF) 178 Page - Dallas Semiconductor

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